1. Field of the Invention
This invention relates to surgical devices for fixing tissue, grafts and sutures to a patient""s bone, and more particularly to biocompatible and bioabsorbable implantable screw-type devices capable of insertion into a bone with reduced torque on the implant and reduced stress on the bone.
2. Description of Prior Art
Many surgical procedures require attachment of tissue, grafts or sutures to a bone or other dense tissue surrounding a bone. For example, various forms of threaded screws have been disclosed for securing a suture in a bore in a bone (see, e.g., U.S. Pat. Nos. 5,209,753, 4,632,100). As another example, interference-fit type screws have been developed for driving into an endosteal bore to secure a bone graft such as in practiced in anterior cruciate ligament reconstruction (ACL) procedure. Such interference-fit screws and ACL methods are disclosed, for example, in U.S. Pat. Nos. 5,139,499, 5,139,520 and 5,234,430.
In the placement of anchor screws or interference-fit screws into a bore, the surgeon typically must apply a high level of torque to drive the screw inwardly to provide a secure attachment. If the endosteal bore is over-sized, any screw-type fixation device will be susceptible to loosening and failure. In driving an interference-fit screw in ACL reconstruction, a high level of torque typically is required which can cause an interference-fit screw to self-tap in an incorrect direction or to diverge from the axis of the endosteal bore. Insertion of such an interference-fit screw also may compress a graft undesirably or cause the bone graft to migrate in the bore, all of which are undesirable.
For both anchor screws and interference-fit screws, surgeons preferably would utilize a bioabsorbable implant fixation body. However, both anchor and interference-fit screws of a bioabsorbable material would be susceptible to shear failure or chipping of the threads during insertion into an endosteal bore, due to the lower strength of bioabsorbable material as compared to a metal alloy screw. There is, therefore, a need for improved anchor screws and interference-fit screws that can be driven with significantly less force, and particularly for such fixation devices that can fabricated of bioabsorbable materials.
The present invention comprises a two-component helically-threaded implantable fixation device for anchoring a graft, suture or tissue in a bore formed in a bone mass (endosteal bore, hereafter) that is actuatable between a first insertion configuration an second anchor configuration. The novel fixation body disclosed herein can be made of a biocompatible metal alloy, or partly or entirely of a bioabsorbable material.
The fixation device of the present invention utilizes a threaded fixation body, hereafter also called an offset helix fixation body, that extends along a first central longitudinal axis. The fixation body is formed of first and second helically-mating members. The fixation body typically defines at least one helical thread about an exterior of the body. The first and second members of the fixation body mate along a constant-lead helical interface, with such a helical interface defined by a radial extending from a second axis that is offset from the above-described first axis of the fixation body. By the term offset, it is meant that the second axis of the helical mating interface is angled from 1xc2x0 to 20xc2x0 or more from the first axis of the body. Alternatively, the second axis is parallel to, but laterally offset from, the first body axis. In this regard, the first and second members of the fixation body can travel helically relative to one another along the helical mating interface between a first insertion configuration and a second anchor configuration. In the insertion configuration, the screw""s outer periphery has a first (lesser) transverse sectional dimension to allow reduced torque in helically driving the screw into an endosteal bore. After the screw body is driven into the endosteal bore, the first and second members are moved helically relative to one another a slight amount about the helical mating interface to the anchor configuration in which the screw""s outer periphery is expanded radially outward providing a second (greater) transverse sectional dimension to thereby secure the fixation device in the bore. Such a screw can be driven into a bore with substantially reduced forces due to the lesser-dimensioned insertion periphery of the body. The final anchoring of the fixation device in the bore also is accomplished with reduced overall torque because only slight relative helical travel of the first and second members is required to move the body from the insertion configuration to the anchor configuration. Of particular interest, since offset helix screws use reduced torque for driving and are actuatable to anchor configuration, such screws can be made of bioabsorbable materials having less tensile strength than conventional metal alloy screws.
In general, the present invention advantageously provides surgical fixation screws and methods for anchoring tissue, grafts and other materials in an endosteal bore in a bone that requires reduced torque for driving into a bore.
The present invention advantageously provides a surgical fixation body made of first and second mating members that mate along a helical interface having an axis that is offset from an axis of the fixation body to provide an insertion periphery and an anchor periphery that have lesser and greater transverse sectional dimensions, respectively.
The present invention provides a fixation body having a first lesser-dimensioned insertion periphery for ease of driving into an endosteal bore and a second greater-dimensioned periphery for anchoring the screw in the bore.
The present invention provides a fixation body having an anchoring periphery that has an increasing transverse dimension in the distal direction for securing the screw body in a bore.
The present invention advantageously provides a fixation screw that requires reduced torque for initial insertion into an endosteal bore because only relatively shallow threads are cut in wall around the endosteal bore.
The present invention provides a bone screw that requires less torque in the final anchoring phase because only slight relative helical travel of the first and second members is required to move the members from the insertion configuration to anchor configuration.
The present invention advantageously provides a fixation screw in which reduced driving torque during insertion allows the screw threads to be formed of bioabsorbable material, and such threads will not tend to fracture or chip during the insertion phase.
The present invention advantageously provides a fixation screw made of helically mating bioabsorbable members (i) having a helical thread with flattened lands in an initial insertion configuration for driving into a bore, and (ii) having a partially sharp edge in an anchoring configuration for cutting into the bone mass as the screw is anchored.
The present invention advantageously provides a fixation screw that can be cannulated for guiding over a guide rod.
The present invention advantageously provides an interference screw and method for a bone-tendon-bone graft in anterior cruciate ligament (ACL) reconstruction in which the interference-fit screw of the present invention will not cause the bone graft to migrate in the endosteal bore.
The present invention advantageously provides a fixation screw with a blunt nose for use in ACL reconstruction in which the screw will not self-tap or diverge from the axis of the endosteal bore.
Additional advantages and features of the present invention will be apparent in the following description, the accompanying drawings and the appended claims.